It was the early 90s, and I was working as a sound engineer and technician at an unusual, prestigious but small recording studio in Canada. After enduring the crazy 70s and 80s, with all its rapid but not always improved change, the flood of early transistor gear, the horrible beginnings of CDs, the ridiculous claims of stereo-makers and speaker companies, the dust had begun to settle among those with the requisite decades of experience and mixed results.

Tube amps for musical instruments stubbornly remained supreme.
By now classic tube designs had been successfully resurrected into niche hi-fi markets, and in the studio, legendary gear had begun to be bought and sold as 'essential', from rare Neumann tube microphones, to the hands-down winner, the Macintosh power amps, along with exotic pieces like tube preamps and plate-reverbs.

The problem at first in the power-amp field was the bottomless craving for more power, and somehow in the transistor shuffle, the lessons of the early monster tube-amps had been lost and forgotten.

Now items like Macintoshs had become so pricey that people came to me complaining, demanding someone make something as good. The glove was thrown down. It had erked me and intrigued that the Macs had proprietary transformers and 'secret' circuitry.

My scientific brain demanded that no 'magical thinking' was allowed. If great amps could be made, they could be made with ordinary parts. They shouldn't and didn't need exotic devices or gold-plated wires, or special magic weights on top of my speakers.
Challenge Accepted:

I thank God that I had a very understanding employer at the time, who was willing to allow some experiments, and front some investment cash for parts etc., as part of their admirable ethic of supporting the arts, even in the face of the rampant flakeness that flapped about everywhere one looked.

Just prior to my plunge, I had had my interest in tube circuits rekindled by another kind musician, who was also convinced about tubes, but who was as poor as me, and who wanted help rebuilding / repairing his ST70.

I quickly figured out that the ST70 was a piece of crap, that needed redesigning from the bottom up. The EL34s were unsuitable for high fidelity, the preamp section was trash, the power-supply circuit was noisy, and even the power transformer was hopelessly underpowered for the job of running a stereo class-A amp.
We rebuilt it and I became addicted to Glass Audio and Speaker-builder, and began to meet a whole pile of people whom I never suspected existed.

Among the various intriguing but always ultimately unsatisfying articles and attempts at producing tube Hi-Fi, I ran into Kimmel's breathtaking work. Without ever knowing it, he led me to the Fountain of Youth of tube circuits, the Mu-Follower. This was the underappreciated gold lurking under everyone's noses.

I unashamedly plagarized and adopted the Mu-Follower for intense study and analysis. My ears had already told me something extemely important through the headphones: The Mu-Follower, properly used, could produce a perfect copy of the music signal. Suddenly a dream re-awoke in me that had been gnawing away at me for years. A power amp that could perform better than transistors.

But so much had to be overcome: Tube power amps were notorious for their 'color'. The main selling point of tubes was their 'musical' coloring and 'warmth'. But I was already dancing to the tune of a different drummer; I had been immersed in studio for so many years, and was now obsessed with sound purity.

I dove into every power-stage design documented, to grasp the shortcomings and benefits of each. I collected everything published in English back to the 1930s I could get my hands on, seeking for at least one more key, some method to overcome the systemic flaws of output sections. The closest thing to what I was looking for was the Circlotron. It was indeed a brilliant design, appealing to my mathematical topological sense, and its elegant solution to 'crossover distortion' in Class A/B was stunning. But, alas, it did not deliver the combination of power and utter pin-drop clarity that I was seeking.

Again I looked at the Macintosh. It was using a type of feedback circuit within the transformer to squeeze out the distortion. But I wanted much more. I didn't want to have the need to do that at all. All my experience with transformers told me they operated better if they weren't pushed into distortion in the first place. I began to study output transformers more closely. Exotic custom-made transformers were too expensive. There had to be a way to make ordinary transformers do what they were supposed to.

Push-Pull circuits were the only ones that met the power requirements needed by typical audio environments, because SE to get sensible power required exotic high-power expensive tubes. This wasn't any better than expensive transformers! Perhaps worse! You only buy a transformer once.

I studied the transformer distortion more closely, in particular the I.M. distortion; where did it come from? yes the non-linearity of the tubes added harmonic components, and with complex signals these could become progressively more non-musical, less related to the signal. But I could audibly hear far more than mere tube distortion in the speaker. The headphones made it obvious.

But OTL circuits were mostly just rumours in the 90s, and required so many tubes that nothing was saved, and much maintenance was incurred. This was no real solution at all.

PSRR: Real and Imaginary

I began to investigate claims about Power-Supplies and their contribution to distortion. They were the single most common problem for most tube circuits, adding hum and buzz, and modulating the music signal.

Even when Power-Supply noise seemed to be under control, tube amps sounded non-musical and mediocre. The problem wasn't solved, only masked. The more current a circuit drew, the less filtering and the louder the ripple. It came sneaking back in with a vengeance whenever the music demanded power. Power supply sagging was another addition to the problem, with its own non-linearity.

Like many a desperate builder in those days, I threw more capacitance into PS circuits, almost recklessly, trying to eliminate noise. But the law of diminishing returns prevented any cost-effective solution in that direction, and indeed, such methods caused their own problems, with damaging surge-currents and part-failures both diode and cap. But I did finally learn how to build reliable power supplies, with delay circuits and surge-limiters.

The Mu Follower Again:

My ears were drawn back again to the Mu-Follower.
Why was it so pure? Why was it noiseless?

(1) It was a Constant Current circuit. Sure. That explained part of its magic: The Power supply was never asked to do more than it always did. Ripple did not increase with varying music signals. But that wasn't the whole story:

(2) It was a Very High Resistance circuit. Most of any noise voltage appearing in the Power Supply was divided across the high resistance portion of the series-circuit. This second fact was what gave the circuit an almost miraculous PSRR. Most of the noise was dissipated as heat in a non-amplifying part of the circuit off from the signal path.

Because of this, the Mu-Follower is the ideal preamp circuit. Critiques aside, it reproduces and amplifies the cleanest signal possible in a tube circuit. Its only drawback, the requirement of a relatively high impedance load, is no limitation for a preamp: You merely add a high-quality cathode-follower as the next stage.

Output Transformer Blues

I turned back to the Output transformer. I studied its behavior more closely.

A balanced D.C. current in its bifilar primary winding was supposed to do two things:

(1) cancel itself out, preventing any magnetization of the iron core, which would use up precious saturation headroom.

(2) reject 99% of powersupply hum and noise, because this also would be balanced in the primary.

(1) cancel itself out?

I thought about it. This is actually nonsense.
In a Push-Pull circuit, the tube on one side conducts more,
while the tube on the other side conducts less.
Its current, not voltage that magnetizes iron;
the currents must swing significantly from side to side in the primary. This means that these currents are sucking back iron-domains, and if the current wave-form isn't a duplicate of the original signal, power is being used in the transformer not belonging to the signal. Any distortion products are also being distorted also in phase due to hysteresis. To put it bluntly, transformers amplify distortion, adding yet more.
It became obvious to me that one must prevent any distortion from entering the transformer in the first place, at all costs.

(2) reject 99% of powersupply hum?

Now lets imagine a 120 Hz ripple (with harmonics).
While there is no music signal,
and the current is balanced on each side of the primary,
we know each tube is acting like a static resistor (and diode in series).
The same percentage (amplitude) of the ripple voltage appears across each half-winding in the primary.
So far so good.
The voltages and opposing forces cancel:
no PS ripple is transmitted to the secondary,
and no current is induced in the iron core either.

Now suppose the music signal is a 30 Hz bass-note.
For the first two cycles, the music signal unbalances the two tubes:
one conducts more, and the other less current.
The voltage also rises at one plate and falls at the other.
now there is momentarily a "D.C." (slow changing A.C.) across the whole primary.
In slow motion if you will, one tube acts as a large resistor,
and the other acts as a small one.
More current flows down one half-winding, magnetizing the core.
(so far so good, because this is the music signal).
But now the PS Ripple also appears more on one leg than the other,
because of these slowing changing voltage-dividers.
The PS Ripple, modulated by the music signal is pushed into the secondary as an AC voltage.
This happens four times (in alternate directions) every 30 Hz.
But most critically important, it is happening most of the time!

During this low music signal, there is darn little PSRR!
The same rule applies again:
We must never let the PS ripple appear across any part of the primary!

One obvious rule, if there weren't enough reasons already,
is not to include a transformer in a multistage feedback loop!

What circuit achieves both goals?

The Mu Follower.
It became obvious to me that not only would the Mu-Follower work in the Output stage, it was essential!

I built several amplifiers, on a commission basis during the 90s,
as a result of my findings.
I also discovered that even the tubes chosen were not really critical,
except to meet power and current-handling objectives.
I used tubes from WWII, vintage, NOS, and modern tubes,
all with great results.

I received a very complimentary review by a writer for Stereo Times, as a personal endorsement.

Quote:

ďLet me tell you about an amazing experience I had not long ago. I was sitting in a local recording studio, listening to my friends lay down the tracks for their upcoming CD. The music consisted of a vocalist, stand up bassist, some percussion, and two guitarists; one acoustic and one electric. As I listened to the live mic feeds, and subsequently the master tape playback of this music, I was struck once again by the notion that home playback just doesnít sound like this. Even through the small, modest speakers being used as monitors there was a richness of tone, attack, transparency and effortless high frequency detail that was stunning. There was a sense of live musicians coming through those monitors that escapes even the most expensive home systems Iíve heard. In contrast, most CDís sound "canned," like all the lovely aural information has been truncated, stuffed into a playback medium that makes a two dimensional caricature out of the original sonic event. So, I sighed and enjoyed the experience, realizing I could never bring that sound home with me. Hereís the kicker. I wandered into a room down the hall from the recording studio where (Nazaroo), a technician working for the studio, had set up his newly designed mono block tube amplifiers. The amps were driving a tiny little pair of NHT "1.5s." Iíve heard those NHTs several times before; nice speakers, pretty flat response, low coloration design, but nothing thatís ever lit my fire. For kicks, from a nearby shelf full of CDs I grabbed an Cassandra Wilson CD, loaded it into the non-descript CD player hooked up to the system and pressed play. My jaw dropped. Iíve been trying to scrape me jaw off the floor ever since. What I heard was an incredibly pure sound. So pure that the sound coming from this commercial CD was startlingly close to the experience of listening to my friendís master tapes in the recording studio. There was none of the electronic hash overlaying the music that I was used to hearing through consumer sound systems. Instead there was a harmonic purity, clarity and lushness that I was accustomed to hearing only in the recording studio. I immediately recognized that I was hearing a level of sound quality that Iíve rarely encountered before, even among the heart-stoppingly expensive systems that populate high end audio. And all this coming from amplifiers putting out around 20 watts into a relatively cheap, small pair of speakers. Clearly (Nazaroo)ís amps were doing something special. So I had (Nazaroo) build me a pair of mono block tube amplifiers. These put out approximately 28 watts a side - a little more powerful than the ones I listened to at the studio. They sound like a fairly Ďpowerfulí 28 watts - enough to drive most mini-monitors easily. In my smallish room they have also acquitted themselves quite well in driving my Von Schweikert VR-4 Gen II speakers - a large, full range floor standing speaker with a 90 dB sensitivity and 6 ohm impedance. Hooking up (Nazaroo)ís amplifiers to almost any speaker makes the speaker "disappear" as a sound source. The amplifiers produce a signal that is so clean and transparent that the low-level ambiance of the recording is revealed. Itís like the very air between your speakers is altered, turned into the recorded space of the CD, and inhabited by palpable sonic images of musicians playing their instruments. Listen to a recording of someone playing acoustic guitar. The high strings of the guitar have a life-like sparkle, without any of the electronic edge or etching to the finger picking that is added by cruder amplifiers. This unnatural electronic etching normally obscures the naturally beautiful overtones of the acoustic guitar. (Nazaroo)ís amps remove these electronic impurities, allowing you to hear the differences between every string, and the resonating wood body of the guitar. Hell, it sounds like you can hear right inside the body of the guitar. In classical music recordings, woodwind instruments in the back of the orchestra that are often obscured or homogenized by other audio equipment are rendered clearly, with each instrument being easily identified, and their melodic line effortlessly followed by the ear. You can hear the texture of a reed buzzing in an oboe, and the vibrating column of air emitted by the body of the instrument. And it goes on and on. Each recording Iíve played through (Nazaroo)ís amplifiers has sounded unique, with itís own recorded ambiance, and pallet of mic techniques being disclosed. The sonic signature of individual stand-up basses, classical guitars, pianos, saxophones etc. are unveiled. (Nazaroo)ís amplifiers simply offer a cleaner, clearer sonic path to the original recording than any amplifier I have encountered. Even my very expensive Conrad Johnson Premier 12 monoblock amplifiers are revealed to somewhat color and obscure the sound in relation to (Nazaroo)ís design. The same is true for other well-regarded amplifiers that I have had at my house. I have not yet heard a more transparent audio signal. Plus, when paired to appropriate loudspeakers, (Nazaroo)ís amplifiers deliver among the richest, tightest bass I have heard from any tube design. So, is there anything more I could ask from these little marvels? Well, yes. In (Nazaroo)ís amplifiers I have encountered something marvelous, but not perfect (Iíve yet to encounter *any* perfect audio component). I find that, in relation to my CJ amplifiers and some other good tube designs, (Nazaroo)ís amplifiers can sound a little less fulsome in the upper midrange. Female voices, while startlingly clear and natural, can loose a little of the physical density or presence that I get with my CJs. Same goes for instruments that inhabit or play in the upper registers, such as alto sax or trumpet. On my CJ amps these instruments move a little more air in my room. Also, one must be careful to pair (Nazaroo)ís amplifiers to the right speaker. Iíve had these amplifiers exhibit excellent control over the bass region of some very large speakers. But if the speaker sensitivity drops too low (or the woofer size gets to big), watch out, the bass can get a bit out of control. Hey, weíre only dealing with 28 watts here, so we must be sensible. I should add that the above caveat regarding a thinness in the midrange seems to be a minor point. While I do hear it, many of my audiophile/musician guests who listen to music through these amps do not pick up on any thinness. In pretty much every case where I have A/Bíd (Nazaroo)ís amps against a competitors my guestís have preferred (Nazaroo)ís, feeling they offer a substantially more realistic sound. ďSo, there you go; right now, (Nazaroo)sís amplifiers offer a level of sonic purity that would be hard to match, even by some of the ridiculously expensive amplifiers offered by other high end amplifier designers. If he continues to perfect his design, I feel he will have a product that, at a fair price point, should make other high end manufacturers hang their head in shame. ď

I've used all kinds of output transformers,
from those cobbled from PA amps, to Plitrons, to custom-made, even power-toroids, as well as Hammonds, RCAs from the 60s.

I'm sharing it obviously.
I'll look for some specific examples that I've built,
and give some construction notes too.

I've given the route by which I came to my designs.
I considered patents, for certain innovations, but as you know,
they are very difficult, and hardly worth it.
I've relied on trademark and other methods for protection
of various commercial ideas.

The mu-follower is a useful circuit, but as you say it needs a high impedance load to give its best. It doesn't get this in an output stage.

The AC current in the normal P-P OPT primary is largely balanced by the AC current in the secondary. The DC current is largely balanced by being in opposite directions in the two legs. It is true that ripple IM does not cancel in the same way that ripple itself does. I only realised that myself relatively recently, but I assume it has been known by others for decades.

If you want to go down the inefficient route of keeping DC out of the OPT then I would have thought that a push-pull SRPP pair would work better, combined with better supply rail regulation to eliminate ripple. The SRPP was actually designed to be an output stage!

PS: Ripple IM in any push-pull circuit will add, while ripple itself cancels, so the p-p mu-follower suffers from exactly the same issue as a normal OPT.

Thanks for your VERY enlightening posting.
So far I've only built a few amp's and can be said to be in learning-mode.
With more people sharing their actual experience like you do here the wheel doesn't have to be re-invented every time someone starts down the 'tubed' road.
As to:

Quote:

Originally Posted by nazaroo

I also discovered that even the tubes chosen were not really critical,
except to meet power and current-handling objectives.
I used tubes from WWII, vintage, NOS, and modern tubes,
all with great results.

...I fully agree. I made 2C34 class AB2/PP sound pretty good imo.
At least far better than any sand-equipment I owned in the past and that even includes a Sony TA-5650 VFET amp 3-4 times the power of the 2C34's.
Right now I'm persuading Compactrons (6AD10) to sound as good and I think I'll get there albeit with less power.

Some of the 'rules' you have discovered and applied remind me of other designs, such as the Loftin-White. This is a circuit that with the ultra-path connection to the cathode of the output tube removes any PSU noise from the OPT.

And a well designed power supply would ideally have an output impedance that is flat over the frequencies of interest - those that don't benefit most from a constant current draw as you have also determined.

Still, I'm not sure I see why the mu-follower is superior to all-else ? -- what I do see is that you have removed the OPT from being a load to the output tubes and replaced it with an active load. Are not other types of active load likely to bring the same 'benefits' as the mu-follower ?

__________________"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Here is the circuit for the WWII version of my amp, one of the first I built.
You can see why it is the cleanest of sounds:

Only two stages. Less than most preamps!

Two transformers.

3 caps.

It is an elegant solution to sonic purity.

Here are some notes:

I used Jensen transformers for the input;
any good output transformer will do.

It requires two heater supplies, one for the upper tubes, one for the lower.
Either float the upper or DC offset it from the powersupply at around 300 volts.

It requires two power supplies, 400-500 for the 1st stage, 700-800 for the 2nd.
The PSRR is so good, you only need simple, modest PS circuits.
You can run the 1st stage off a resistor/cap leg from the main B+.

The top tube was a 5998 in the power section (V3 A, V3 B).
You can also try a couple of 6L6s or 6V6s, in triode-mode.
The bottoms were a 1625s. 807s are equivalents (triode mode with the usual precautions)
6SN7s or 12AX7s will do in the first stage.

The mu-follower is a useful circuit, but as you say it needs a high impedance load to give its best. It doesn't get this in an output stage.

The AC current in the normal P-P OPT primary is largely balanced by the AC current in the secondary. The DC current is largely balanced by being in opposite directions in the two legs. It is true that ripple IM does not cancel in the same way that ripple itself does. I only realised that myself relatively recently, but I assume it has been known by others for decades.

If you want to go down the inefficient route of keeping DC out of the OPT then I would have thought that a push-pull SRPP pair would work better, combined with better supply rail regulation to eliminate ripple. The SRPP was actually designed to be an output stage!

PS: Ripple IM in any push-pull circuit will add, while ripple itself cancels, so the p-p mu-follower suffers from exactly the same issue as a normal OPT.

At one point, the man who reviewed the amps was still in shock,
so he set up a series of A/B tests, and drove me and the amps all over Ontario.

At one particular location, a fellow had spent several thousand in parts,
for an elaborate SE amp,he had designed using very expensive transformers, silver wire etc., and some $10,000 pair of speakers.

He played me his rig for about an hour.
I had to admit it was beautiful, rich and warm, quite an accomplishment.
I said nothing, and neither did the reviewer.
We set up my monoblocks quietly, then spent the next 3 hours listening to everything he could throw at them.
It really sounded like Diana Krall et al, were in the room, saxes, accoustic basses and all.

He said the monoblocks were incredible.
I felt kind of sad that my amps had defeated his greatest efforts and expense.
We parted friends.

In a similar A/B shootout with Bryston's best and purest solid-state offerings,
we blew their amps out of the water.

It was a thrilling experience, never to be repeated.
I felt dishonest in a way, because I knew a key point of my innovation was based on the work of others.

My scientific brain demanded that no 'magical thinking' was allowed. If great amps could be made, they could be made with ordinary parts. They shouldn't and didn't need exotic devices or gold-plated wires, or special magic weights on top of my speakers.

That I can agree with. Get the circuit right, and then component selection is the icing on the cake. Far too many people want to make a cake out of icing!